1. Field of the Invention
The present invention relates to a method for forming an image for eliciting an electrostatic latent image in electrophotography.
2. Description of the Related Art
In recent years, in the field of electrophotography, some currents of technologies have arisen in the light of miniaturization of a device, cost-effectiveness, environmental grounds, and so on. One of them is a technology called a cleaning simultaneous with development or cleanerless.
In the conventional electrophotographic process, a residual toner remained on a latent image bearing member after transferring toner onto a recording medium. The toner is then removed therefrom at a cleaning step by anyone of various methods, wherein the removed residual toner is accumulated into a waste toner container as waste toner. A method for image forming, in which the above steps were repeated through the cleaning step, have been used. For such a cleaning step, conventionally, blade cleaning, fur brush cleaning, roller cleaning, and so on have been used. These methods are designed to scratch the residual toner forcefully, or to dam back and recover the residual toner into a waste toner container. Therefore, there is a problem caused by pressing such a member used for cleaning the residual toner against the surface of the latent image bearing member. For example, the member being strongly pressed against the latent 1 image bearing member causes wearing of the latent image bearing member and shortening of life thereof. From the point of view of the device, the installation of a cleaning mechanism into a device inevitably leads to enlargement of such a device, hindering miniaturization of the device. Furthermore, a system that does not generate waste toner while having excellent fixation and offset-proof properties has been desired from the viewpoint of saving resources, reduction in waste and effective use of toner.
On the other hand, as a system that does not generate waste toner, technologies called cleaning simultaneous with development or cleanerless have been also proposed in the art. For example, the technologies about cleanerless have been disclosed in JP 59-133573 A, JP 62-203182 A, JP 63-133179 A, JP 64-20587 A, JP 2-302772 A, JP 5-2289 A, JP 5-53482 A, JP 5-61383 A, and so on.
Furthermore, there is a technology of contact charging as an ecology technique about charging.
In electrophotography, a typical method for forming an electrical latent image is one comprising allowing uniform charging to a predetermined polarity and potential on the surface of a photoconductor utilizing a photoconductive material as a latent image bearing member and subjecting the charged photoconductor to an image-pattern exposure to form an electric latent image.
Conventionally, a corona charging device (a corona discharging device) has been used frequently as a charging device that carries out a charging treatment (also including an electric discharge treatment) on the surface of a latent image bearing member to charge it uniformly with a desired polarity and potential. The corona charging device is a non-contact charging device and includes a discharge electrode such as a wire electrode and a screening electrode surrounding the discharge electrode. Furthermore, the corona charging device has a discharge opening being formed to face an image bearing member which is provided as an object to be charged. The surface of the image bearing member can be charged to a desired polarity and potential by subjecting the surface to a discharge current (a corona shower) which is generated by the application of a high voltage to both the screening electrode and the discharge electrode.
In recent years, various kinds of contact charging devices have been proposed and put in practical use as charging devices for objects to be charged such as a latent image bearing member because of their advantages such as low generation of ozone and a low requirement on electric power, compared with a corona charging device.
A contact charging device is to charge the surface of an object to be charged to desired polarity and potential by bringing a conductive charging member (a contact charging member or a contact charging device) such as a roller type (charging roller), a fur brush type, a magnetic brush type, and a blade type into contact with the charging object such as an image bearing member to allow the application of a predetermined charging bias to the contact charging member.
In the charging mechanism (the mechanism of charging, and the principle of charging) of the contact charging, two kinds of charging mechanisms: (1) a discharge-charge mechanism; and (2) a direct-injection charging mechanism, are intermingled, and each characteristic appears depending on which mechanism is dominant in the contact charging.
(1) Discharge-charge Mechanism
It is the mechanism in which the surface of a charging object is charged according to the discharge phenomenon produced in a minute gap between the contact-charging member and the object to be charged. The discharge-charge mechanism has the fixed discharge thresholds of the contact-charging member and the object to be charged, so that there is a need to apply a voltage larger than the charging potential to the contact-charging member. In addition, even though the amount of the resulting discharged product is remarkably small as compared with a corona charging device, theoretically, the generation of the discharged product is hardly avoidable. Thus, a trouble to be caused by an active ion such as ozone will be inevitable.
(2) Direct-injection Charging Mechanism
It is a system in which the surface of an object to be charged is charged by directly injecting an electrical charge into the object to be charged from the contact-charging member. Alternatively, the mechanism may be called a direct charging, injection charging, or charge-injection charging. In more detail, the contact-charging member of an intermediate resistance contacts the surface of the object to be charged and injects electrical charges directly into the surface of the object to be charged. At this time, basically, a discharge phenomenon is not used (i.e., discharge does not occur). Therefore, even if the applied voltage to the contact-charging member is equal to or below a discharge threshold, the object to be charged can be charged to the electric potential corresponding to the applied voltage. As this charging system does not involve the generation of ions, there is no trouble to be caused by the discharged product. However, because of the properties of the direct-injection charging, the contact ability of the contact-charging member with the object to be charged is greatly effective against the charging property. Therefore, in order that the contact-charging member is constructed such that it is brought into contact with the charging object at a higher frequency, there is a need of designing the contact-charging member to have denser contacting points with the object to be charged, to make the difference in rotating speed between the contact-charging member and object to be charged larger, and so on.
For the contact-charging device, a roller charging system using a conductive roller (a charging roller) as a contact-charging member is preferable in respect of the stability of charging and is widely used.
In a charging mechanism used in the conventional roller-charging, the discharge-charge mechanism of the above item (1) is dominant.
A charging roller is produced using a rubber or foam material which is conductive or has an intermediate resistance. In addition, some rollers are constructed by laminating the materials so as to have desired characteristics.
Furthermore, the charging roller has its own elasticity so as to have a constant contacting status with the charging object. Therefore, the frictional resistance thereof is large. In many cases, furthermore, the charging roller is driven by the object to be charged or with a little speed difference therewith. Therefore, even if direct-injection charging is about to be carried out, a decrease in absolute charging ability, an insufficient contact ability, contact unevenness attributed to the form of the roller, and charging unevenness due to deposit on the object to be charged are unavoidable.
FIG. 1 is a graph that represents an example of charge efficiency of the contact charging in the electrophotographic method. The horizontal axis indicates a bias applied to the contact-charging member and the vertical axis indicates the charged potential of the object to be charged (hereinafter, also referred to as a photosensitive member) obtained at the time. The charging characteristics of the photosensitive member when the roller-charging is used are denoted by the letter “A”. The charging is initiated at a potential over a discharge threshold of about −500 V. Therefore, for charging the photosensitive member at −500 V, typically, the application of a DC voltage of −1,000 V or the application of an AC voltage with a peak-to-peak voltage of 1200 V so as to constantly keep the potential difference equal to or more than the discharge threshold in addition to the DC charging voltage of −500 V is commonly performed to converge the potential of the photosensitive member to the charged potential.
More specifically, in the case where a charging roller is brought into contact with an OPC photosensitive member of 25 μm in thickness by pressurizing, when the voltage of about 640 V or more is applied, the surface potential of the photosensitive member will begin to rise. After rising, the surface potential of the photosensitive member increases linearly with an inclination of 1 with respect to the applied voltage. Here, this threshold voltage is defined as a charging-initiation voltage Vth.
In other words, for providing the photosensitive member with the surface potential Vd to be required in the electrophotographic method, the charging roller requires that a DC voltage which is equal to or higher than the sum of the surface potential and the charging-initiation voltage (Vd+Vth) is applied. Thus, a charging, method in which the charging is performed by applying only DC voltage to the contact-charging member is referred to as “a DC charging system”.
However, in the DC charging system, the resistance value of the contact-charging member varies as its environmental conditions, etc. are changed. In addition, the thickness of the photosensitive member is changed as the photosensitive member is shaved, so that the Vth of the contact-charging member can be also fluctuated. Therefore, it is difficult to adjust the potential of the photosensitive member to a desired potential.
For this reason, as disclosed in JP 63-149669 A, the “AC charging system” has been used for attaining further equalization of charging. The “AC charging system” applies to the contact charging member a voltage obtained by superimposing the AC component of the peak-to-peak voltage of 2×Vth or more on the DC voltage corresponding to the desired Vd. This aims at “equalizing effects” of the potential with AC. Therefore, the potential of the object to be charged is converged on the Vd in the middle of the peak of the AC voltage. The potential is not influenced by any disturbance from its surroundings such as environmental one.
However, even in the contact-charging device, its essential charging mechanism utilizes the discharge phenomenon from the contact-charging member to the photosensitive member. Therefore, as described above, the voltage to be applied to the contact-charging member should be equal to or higher than the surface potential of the photosensitive member, and a trace amount of ozone is generated.
When the AC charging is performed for charge equalization, further generation of ozone, the generation of oscillation noises from the contact-charging member and the photosensitive member in the electric field of the AC voltage (AC charging noise), a deterioration in the surface of the photosensitive member due to discharge, and the like occur remarkably, thereby causing new problems.
Furthermore, the fur-brush charging uses a member (a fur brush-charging device) having a brush part constructed of conductive fibers as a contact-charging member. The conductive fiber brush part is brought into contact with the photosensitive member provided as an object to be charged to charge the surface of the photosensitive member to the desired polarity and potential by applying a predetermined charging bias. The discharge-charge mechanism of the above item (1) is dominant in the charging mechanism of the fur-brush charging.
For the fur brush-charging device, there are two different types, namely a fixed type and a roll type, which have been practically used in the art. The fixed type fur brush-charging device is constructed such that fibers of intermediate resistance are woven in a base fabric in the shape of a pile and are then fixed on an electrode. On the other hand, the roll type one is constructed such that a pile is twisted around a core metal. In this case, the fur brush-charging device having a fiber density of about 100/mm2 can be prepared with comparative ease. However, the contact ability is still inadequate for attaining sufficiently uniform charging by direct-injection charging. In addition, for attaining sufficiently uniform charging with direct-injection charging, it is necessary to differ the rotating speed of the fur brush-charging device from that of the photosensitive member, which is hardly attained by the mechanical configuration thereof and is not realistic.
The charging characteristics of the fur brush-charging at the time of applying the DC voltage can be represented as shown by “B” in FIG. 1. Therefore, in the case of the fur brush-charging as well, the charging Is often performed using a discharge phenomenon with a high charging bias in both the fixed type and the roll type of the fur brush-charging.
On the other hand, magnetic brush charging uses a member (a magnetic brush charging device) having a magnetic brush part as a contact-charging member, in which conductive magnetic particles are magnetically trapped into a brush shape by a magnet roll or the like. The magnetic brush part is brought into contact with the photosensitive member provided as an object to be charged to charge the surface of the photosensitive member to the desired polarity and potential by applying a predetermined charging bias.
In the case of magnetic brush charging, the direct-injection charging mechanism of the above item (2) is dominant in the charging mechanism.
The conductive magnetic particles that constitute the magnetic brush part are those having grain sizes in the range of 5 to 50 μm. In addition, providing the magnetic brush charging device with a sufficient rotating speed different from that of the photosensitive member allows uniform direct-injection charging.
As is represented by “C” in the graph of the charging characteristics of FIG. 1, it becomes possible to obtain the charged potential almost proportional to the applied bias.
However, in this case, there are several disadvantages such as a complicated configuration of the device and the adhesion of conductive magnetic particles composing the magnetic brush part, which have fallen to the surface of the photosensitive member.
Here, the case is considered in which these contact-charging methods are applied in the cleaning simultaneous with development method or the cleanerless image forming method as described above.
The cleaning simultaneous with development method or the cleanerless image forming method does not use a cleaning member. Thus, the transfer residual toner on the photosensitive member directly contacts the contact-charging member, so that the toner may be adhered to or mixed in the contact-charging member. Furthermore, in the case of the charging method in which the discharge-charge mechanism is dominant, the adhesion property of the toner with respect to the charging member becomes worse due to toner deterioration caused by the discharge energy. When the insulating toner typically used in the art is adhered to or mixed in the contact-charging member, the charging property of the object to be charged is degraded.
In the case of the charging method in which the discharge-charge mechanism is dominant, such degradation in the charging property of the object to be charged occurs suddenly at the time when the toner layer adhering to the surface of the contact-charging member becomes a resistance that blocks the discharge voltage. On the other hand, in the case of the charging method when the direct-injection charging mechanism is dominant, the transfer residual toner adhered to or mixed in the contact-charging member reduces the contact probability of the surface of the contact-charging member and the object to be charged, thereby degrading the charging property of the object to be charged.
The degradation in the uniform charging property of the object to be charged leads to a degradation in contrast and uniformity of the electrostatic latent image after the image exposure, resulting in a decrease in the image density while worsening the fog.
Furthermore, in the cleaning simultaneous with development method or the cleanerless image forming method, it is important that the charging polarity and the charge amount of the transfer residual toner on the photosensitive member are controlled to stabilize the recovery of the transfer residual toner in the step of development so that the deterioration of the development characteristics due to the recovered toner is prevented. Therefore, the charging member is responsible for controlling the charging polarity and the charge amount of the transfer residual toner.
The behavior of the toner before and after the step of image transfer will be described with reference to the example using a common laser printer. In the case of a reversal development using a charging member that applies a negative polarity voltage, a photosensitive member having a negative charging property, and toner having negative charging property, a visualized image is transferred onto a recording medium by a transfer member having a positive polarity. Here, the charging polarity of the transfer residual toner varies from positive to negative depending on, for example, the relationship between a type of recording medium (difference in thickness, resistance, dielectric constant, etc.) and image area. However, even if the transfer residual toner together with the surface of the photosensitive member are shifted to the positive polarity side in the step of transfer, due to the charging member having a negative polarity at the time of charging the photosensitive member having the negative charging property, the charging polarity of the transfer residual toner can be uniformly set to the negative side. Therefore, in the case of using reversal development as a developing method, the negatively charged transfer residual toner remains on a bright section potential part where the toner should be developed. In this case, on the other hand, on the dark section potential part where the toner should not be developed, the transfer residual toner is pulled toward the toner carrying member in relation to a developing electric field and can be recovered without remaining on the photosensitive member having a dark section potential. Therefore, the cleaning simultaneous with development or the cleanerless image forming method are achieved by controlling the charging polarity of the transfer residual toner simultaneously with charging property of the photosensitive member by the charging member.
However, it becomes difficult to recover the toner by the developing member when the amount of the transfer residual toner adhered on or mixed in the contact-charging member exceeds the amount in which the contact-charging member can control the charged polarity of the toner because the charged polarities of the transfer residual toner cannot be set uniformly. In addition, the charging properties of toner on the toner carrying member may be affected when the uniform charging is not achieved over the transfer residual toner even though the transfer residual toner is recovered on the toner carrying member by mechanical force such as sliding friction. Thus, the development characteristics may be decreased.
In other words, in the cleaning simultaneous with development or the cleanerless image forming method, the charge-control characteristics when the transfer residual toner passes through the charging member and the characteristics of adhering on or mixing in the charging member are related closely to durability and image quality.
In terms of the adhesion and mixing characteristics of the transfer residual toner to the charging member, many techniques relating to the charging process have been disclosed.
Disclosed in JP 7-99442 B is the configuration in which powders are applied on the surface of the contact-charging member, of which the surface is in contact with the surface of the object to be charged, for preventing the charging unevenness and providing uniform charging in a stable manner. However, the rotation of the contact-charging member (charging roller) is driven by the object to be charged (photosensitive member) (no driving with speed difference). Even though the generation of the ozone product is extremely decreased as compared with the corona charging device such as a scorotron, the charging principle is still based on the discharge-charge mechanism just as in the case of the roller-charging described above. In particular, for obtaining more stable charging uniformity, the application of the voltage is performed such that the AC voltage is superimposed on the DC voltage. Thus, the generation of the ozone product by discharge may be increased. Therefore, when the device is used for a long time, the problem such as an image flow caused by the ozone product tends to occur. Furthermore, when it is applied to the cleanerless image forming apparatus, it becomes difficult to adhere the applied powders uniformly on the charging member because of the mixing of the transfer residual toner, so that the effect of allowing uniform charging becomes decreased.
In JP 5-150539 A, there is disclosed a method for image forming using contact charging, in which toner includes at least image-manifesting particles and conductive particles having an average particle size smaller than that of the image-manifesting particles, for preventing the charge inhibition to be caused by adhesion or accumulation of toner particles or silica fine particles which could not be removed by blade cleaning on the surface of the charging means after repeating image formation in the long term. However, the contact charging or the adjacent charging used herein is based on the discharge-charge mechanism, so that there arises the problem resulting from not the direct-injection charging mechanism but the discharge-charging as described above. In the case of application to the cleanerless image forming apparatus, as compared with one having a cleaning mechanism, an influence on the charging property, which is caused by a large amount of conductive fine particles and transfer residual toner undergoing the charging process, and the recovering property with respect to a large amount of the conductive fine particles and the transfer residual toner in the development process, and an influence on the development characteristics of toner with the recovered conductive fine particles and the transfer residual toner are not considered. Furthermore, in the case of applying the direct-injection charging mechanism on the contact charging, a required amount of the conductive fine particles is not supplied to the contact-charging member, so that the charging failure may be caused due to an influence of the transfer residual toner.
Furthermore, in the case of the adjacent charging, it is difficult to uniformly charge the photosensitive member in the presence of a large amount of the conductive fine particles and the transfer residual toner. There is no effect of leveling the pattern of the transfer residual toner, so that a pattern ghost for shielding the pattern image exposure of the transfer residual toner will be caused. Furthermore, upon the instantaneous interruption of a power source or a paper jam during the image formation, the contamination inside the device with toner becomes remarkable.
Furthermore, disclosed, for example, in JP 2001-188416 A, JP 2001-215798 A, and JP 2001-215799 A, is a method of image forming with cleaning simultaneous with development, in which the transfer residual toner recovering property in the development is assisted or controlled using a roller member, fur brush or the like to be contacted against the photosensitive member or the charging member during a period between the transferring process and the charging process. Such a kind of the Image forming apparatus has a favorable cleaning-simultaneous-with-development property and is capable of extensively decreasing the amount of waste toner. In this case, however, the advantages of the cleaning simultaneous with development are impaired in that its cost becomes high and it cannot be designed to be smaller.
On the other hand, for example, in JP 10-307456 A, JP 10-307421 A, JP 10-307455 A, JP 10-307457 A, JP 10-307458 A, and JP 10-307456 A, there is disclosed a method for forming an image with cleaning simultaneous with development, in which the conductive particles are directly applied to the charging member with specific grain size or are continuously supplied to the charging member in an indirect manner by externally adding the conductive particles to the toner. In these methods, at the initial stage of printing, a good image can be obtained without causing at least defective charging and light shielding upon the image exposure. Therefore, regarding the above proposal, further improvements have been required and possible in the performances when toner particles having smaller particle size are used for improving the stability in long-term repetitive usage and increasing a resolution.
Furthermore, even though there is a need for improvement of toner in consideration of transfer, charging, and recovering properties, in the prior art, there is no description about a preferable configuration of toner and no consideration with respect to durability and charging stability against the change in printing ratio, resulting in the insufficient ones.
For example, in each of JP 59-133573 A, JP 62-203182 A, JP 63-133179 A, JP 64-20587 A, JP 2-302772 A, JP 5-2289 A, JP 5-53482 A, JP 5-61383 A, and JP 2001-194864 A, there is no description about a favorable method of image forming. In addition, there is no description about the configuration of toner.
In JP 2001-188416 A, JP 2001-215798 A, JP 2001-215799 A, and so on, there is proposed a contact-charging cleanerless system using a two-component developing system. In this proposed system, effects can be surely obtained to a certain degree with respect to charging defect. However, the photosensitive member originally tends to be chipped by sliding friction with the ears of carriers in the two-component development. Since it is easy to generate especially the half-tone unevenness resulting from a deep blemish or the like, a further improvement also from the viewpoint of the photosensitive member service life and so on is needed.
Furthermore, as disclosed in JP 2000-181200 A, another system is proposed such that the polarity of toner is controlled by making a toner-scraping member contact to the charging roller to increase the toner recovering ability. With this method, it is surely possible to improve the toner recovering ability at an initial stage of the process. Even though toner recovering ability is improved, there is a need that the residual toner passes through the gap between the charging member and the toner image bearing member. Therefore, there is a tendency of causing aggregation and fusion of toners. In other words, as It results in the occurrence of light shielding, fusion, and so on, a further improvement is required.
The charge control characteristics of the transfer residual toner when it passes through the charging member are improved to enhance the cleaning-simultaneous-with-development performance as disclosed in JP 11-15206 A. That is, there is proposed a method of image forming using toner including toner particles containing a specific carbon black and a specific azo-based iron compound and inorganic fine particles. Furthermore, in the method of image forming with cleaning simultaneous with development, it is also proposed that the cleaning simultaneous with development performance is improved by decreasing the amount of the transfer residual toner with toner excellent in transfer efficiency which specifies the shape factor of toner.
However, the contact charging used here is also based on the discharge-charge mechanism, and has the above-mentioned problem caused not by direct-injection charging mechanism but by discharge-charging. Furthermore, these proposals attain the effects of suppressing a decrease in the charging properties of the contact-charging member in the presence of the transfer residual toner. In this case, however, the effects of positively increasing the charging property are not expectable.
Furthermore, in JP 2001-235897 A and JP 2001-235899 A, there is disclosed a method in which toner is used, which can improve the wear resistance of the surface of the photosensitive member by having no magnetic substance on the surface of the toner and is superior in transfer property and rigidity because of a specific circularity, in the adjacent or contact development method. In this method, the amount of the transfer residual toner is small, so that the inhibitory affect on the charging part is small and the recovering ability in the developing part is also excellent. In this case, however, the conductive particles on the surface of the toner tend to be peeled off because of its excellent fluidity. As a result, there newly causes another problem in that a decrease in the amount of conductive particles to be supplied is easily caused in the latter stage of the durability. In addition, the charging property is retained as the amount of the toner present in the charging part is extremely small. Therefore, toner contamination of the photosensitive member supposedly occurs by the generation of the so-called jam or the like, also in the processes subsequent to the transferring process. In such a case, variations of resistance on the charging part, inroads of toner, and so on are increased. As a result, recovery of the image from the charging defect status takes much time. Furthermore, there is a tendency of causing streak-like fog and unevenness on the half-tone image.
Furthermore, in recent years, there is a tendency of increasing the degree of toner fog resulting from insufficiency of the charging property, and the amount of the transfer residual toner, and widening the toner charging distribution due to the increasing requirement for the high Image quality along with a smaller toner particle size and an increase in print speed. However, there is no satisfactory toner having an appropriate developing property and the recovering ability or cleanerless image forming method, while considering the above facts.